1. Field of the Invention
The present invention relates to an illuminance controller of a light source and an endoscope including the same, which is capable of controlling an illuminance of a light beam in an accurate and ready manner without causing any problem such as an overshoot, a hunting or a responsive lag, and producing an uniform illuminance distribution of a light beam over an entire illuminated area.
2. Description of the Prior Art
An endoscope has been widely used for a medical treatment or inspection of organs such as the duodenum, the rectum, the colon, the esophagus, the stomach and so forth.
A conventional electronic endoscope comprises a scope for observing the inside of an internal organ, a light source for generating a light beam, and an operational unit for operating picture signals picked up by the scope. The scope includes an end portion having an optoelectronic photographing element such as a charge coupled device (CCD), and an optical system for observing an object, a flexible intermediate portion having a light guide composed of optical fibers and electric wires therein, a manual operating handle portion. The light source includes a light source lamp for generating the light beam and a condenser lens for collecting the light beam emitted by the lamp to feed the light beam to the light guide.
In this case, when a luminous energy or an illuminance of a light beam to be fed to the light guide is too much, a halation or blooming arises in an observing area of the object to be observed by the CCD, and, when the illuminance of the light beam is too small, it is difficult to ensure a sufficient observing visual filed. Therefore, during an operation using the endoscope, the illuminance of the light beam to be fed to the light guide is controlled to a proper intensity.
In FIGS. 1 and 2, there is shown a conventional illuminance controller for use in a light source of an endoscope, and the illuminance controller comprises a pair of shutter plates 1 and 2 movably mounted between a condenser lens 3 and a rear end 5 of a light guide 4. The shutter plates 1 and 2 are slidably moved in opposite directions indicated by an arrow a in FIG. 1, perpendicular to a light path of a light beam 6 generated by a lamp (not shown), for partially or entirely opening or closing the light path of the light beam 6 to control the illuminance of the light beam 6 to be fed to the rear end 5 of the light guide 4. The shutter plates 1 and 2 functions as a diaphragm or a lens stop.
In FIGS. 3 and 4, there is shown another conventional illuminance controller, like the controller shown FIG. 1, having a similar construction thereto, except that a fishtail-like shutter plate 7 is movably mounted between the condenser lens 3 and the rear end 5 of the light guide 4. The fishtail-like shutter plate 7 is moved in a direction indicated by an arrow b in FIG. 3 so as to open or close the light path of the light beam 6 for controlling the illuminance of the light beam 6 in the same manner as the previous embodiment of FIG. 1. The fishtail-like shutter plate 7 acts as a diaphragm or a lens stop as well.
However, in the conventional illuminance controllers described above, the diaphragm is adapted to adjust the illuminance of the light beam corresponding to a rotation of a drive motor. Accordingly, the variation rate of the illuminance of the light beam largely depends on a rotative angle .theta. of the drive motor, as indicated by a curve A in FIG. 6. That is, the variation rate of the illuminance of the light beam is represented by .DELTA.I/I wherein I is the illuminance of the light beam and .DELTA.I is the variation of the illuminance corresponding to the rotation of the drive motor. As to the curve A in FIG. 6, when the rotative angle .theta..sub.1 is small, the variation rate .DELTA.I.sub.1 /I.sub.1 of the illuminance becomes large, and, in turn, when the rotative angle .theta..sub.2 is large, the variation rate .DELTA.I.sub.2 /I.sub.2 of the illuminance becomes small.
Accordingly, when the rotative angle .theta. of the drive motor is small, it is liable to produce an overshoot or a hunting, and, when the rotative angle .theta. is large, a responsive lag arises. In both cases, It is quite difficult to suitably adjust the illuminance of the light beam to a desired value.
Further, in the conventional illuminance controllers shown in FIGS. 1 to 4, the illuminance of the light beam to be fed to the light guide 4 is controlled by cutting the light path of the light beam 6 using the shutter plates 1 and 2 or the fishtail-like shutter plate 7 slidably arranged between the condenser lens 3 and the light guide 4, and the light guide 4 emits the light beam 6 only to the same angular direction as the incident angle of the light beam 6. Hence, the control of the illumination of the light beam can be readily carried out, but the distribution characteristics of the light beam such as the light intensity distribution or the illuminance distribution of the light beam emitted from the light guide 4 to the object to be observed becomes uneven over the illuminated area with the variation depending on the cut amount of the light beam. As a result, the desired uniform illuminance may not be obtained in all area of the observed visual field. When the object is observed by using this endoscope, the light intensity remarkably changes in a picture depending on the distance and the area of the object, and it often becomes difficult to observe the object.
In order to remove such a problem, a snail-like rotary shutter plate 8 having a spiral periphery for cutting the light path of the light beam has been proposed, as shown in FIG. 5. In this embodiment, the illuminance of the light beam for illuminating the object is varied in order to maintain the brightness of the picture of the object to a certain value by changing the axial distance between the optical axis of the light beam and the rotary axis of the rotary shutter plate 8 depending on the brightness of the picture, thereby controlling the irradiation time of the pulsed light beam.
In this case, when the luminous intensity of the light beam generated by the light source is constant, the illuminance of the light beam is in proportion to its irradiation time. When the rotational speed of the rotary shutter plate 8 is constant, assuming that the axial distance between the rotary axis of the rotary shutter plate 8 and the optical axis of the light beam is r, the irradiation time is proportional to an opening angle .theta. which is defined by an initial side 8a of the angle .theta. and an angle generating line 8b passing through the center of the rotary shutter plate 6 and an intersection of the spiral periphery of the rotary shutter plate 8 and the circumference of a circle having a radius r around the center of the rotary shutter plate 8.
In the rotary shutter plate 8 shown in FIG. 5, r.sub.0 is the minimum value of the radius r, and r-r.sub.0 is an effective axial distance. The opening angle .theta. of the rotary shutter plate 8 is formed to satisfy a formula .theta.=k(r-r.sub.0), wherein k is constant, as shown by a straight line G in FIG. 18, in which the irradiation time of the light beam is represented by the opening angle .theta.. Therefore, the opening angle .theta. always varies with the constant variation rate, and thus the illuminance, i.e., the irradiation time of the light beam varies in proportion to the axial distance r.
However, in this embodiment, since the opening angle .theta. corresponding to the illuminance or the irradiation time of the light beam is proportional to the axial distance r, when the axial distance r is varied, the variation rate .DELTA..theta./.theta. of the opening angle .theta. is changed largely depending on the axial distance r.
Accordingly, even if the illuminance of the light beam is varied with a certain rate depending on the object to be photographed, the variation amount .DELTA.r of the axial distance r varies in various ways. When the variation amount .DELTA.r of the axial distance r is small, for instance, in case of a pulse motor for driving the rotary shutter plate, a speed reducer having a sufficiently large speed reduction ratio is required. However, in this case, when the variation amount .DELTA.r becomes large, i.e., it is required to rotate the output shaft of the speed reducer at a high speed, it becomes inconvenient.
Further, when the irradiation time of the light beam is automatically controlled in order to maintain the illuminance of the light beam for illuminating the object to the fixed value, if a feedback gain is determined so as to obtain a stable motion when the angle variation rate .DELTA..theta./.theta. is large, it becomes liable to occur a responsive lag when the angle variation rate .DELTA..theta./.theta. is small. Thus, in an automatic illuminance control system, it becomes impossible to prevent as a whole the problems such as the overshoot, the hunting, the responsive lag and the like.